Hypertension is a silent killer. According to the National Health Survey, 1998, about 27.3% of the Singapore population between the ages of 30-69 years are hypertensive. This translates to about 600,000 hypertensives based on the 2.2 million people in this age group, of whom about half have not been previously diagnosed. The prevalence of hypertension and its related complications are on the rise, with:                1. one new hospital admission for stroke every hour;        2. 25% of stroke patients are <45 years old;        3. one heart attack every 3 hours as recorded by the Acute Myocardial Infarction Register;        4. more and younger patients requiring renal dialysis.        
Such facts are not unique to Singapore. Many developed countries have comparable, if not higher, statistics. In other words, hypertension is a global problem of epidemic proportions.
In Singapore alone, there is at least one person coming down with stroke every hour. The numbers are rising year after year. Moreover, death from stroke in Singapore accounts for more than 12% of all deaths since 1996.
Together with heart ailment, it accounts for more than 32% of all deaths since 1996, i.e. more than one-third of all mortalities in Singapore.
Further, every year there are about 27,000 to 30,000 pregnancies leading to successful deliveries. Of these, thousands of pregnant women suffer from a condition called pre-eclampsia. This is a condition whereby the mother suffers from a rise in blood pressure during pregnancy. The blood pressure can rise to dangerous levels without warning and it can lead to convulsion and brain damage to the mother, and sudden intra-uterine death of the baby. The morbidity and mortality of pre-eclampsia is directly related to the level and control of blood pressure of the patient.
The central event linking the 3 major ailments is blood pressure. In fact, in many instances of strokes and heart attacks, the usual and final pathway is a sudden and dangerous rise in blood pressure before catastrophe strikes.
Therefore, the detection and prevention of further rises or falls in the final pathway holds the key to the prevention and reduction of strokes, heart attacks and eclampsia.
Currently, patients who suffer from the above illnesses are monitored either as outpatients or in-patients in a hospital. The majority of these are outpatients. When one visits a doctor, be it monthly or fortnightly, the blood pressure reading is obtained by using a blood pressure cuff sphygmomanometer. They use occlusive methods, i.e. air is pumped into the cuff to occlude the artery and is slowly released to finally allow the blood to overcome the resistance and flow through. A flow turbulence is thus set up and picked up by the doctor listening to it. The blood pressure is then recorded. The self-monitoring devices that are available on the market generally all use occlusive methods, the difference being the turbulence are picked up by various methods, such as via a microphone. In other words, the number of readings is totally dependent on the number of times that the artery is being occluded, whether it is manual or pre-set electronically. The monitoring is therefore not continuous, in the sense of having beat-to-beat readings.
To make matters worse, whenever the doctor detects a normal or “good” blood pressure in his clinic, he usually makes 3 assumptions:                1. the patient's blood pressure from the last test must be “good”;        2. his blood pressure until the next test will be “good”; therefore, he will not have a stroke, heart attack or convulsion as in the case of a pre-eclampsic woman.        
Unfortunately, these assumptions are far from the truth as the above incidents have revealed. Casual blood pressure measurements taken in the doctor's office or by the patients themselves are not necessarily representative of a person's 24-hours blood pressure. Therefore, it would be advantageous to be able to catch the “final pathway” of sudden changes in blood pressure/pulse, by being able to monitor a person's blood pressure continuously and be able to sound the alarm at the right time to prevent a catastrophe.
One method of continuously monitoring blood pressure is suggested in U.S. Pat. No. 5,485,848. That patent purports to disclose a non-invasive and non-intrusive portable device for monitoring a user's arterial blood pressure. However, that device has the disadvantage that it needs to fix a nominal or base pressure by fixing the strap tension. The calibration is also user-specific. It assumes that base pressure can be maintained constant for the calibration to work. It is not practically possible to fix the base pressure of a moving wrist by the methods described. At most, it only keeps the strap circumference constant, instead of keeping the pressure constant. By fixing the circumference of the strap, pressure changes are even greater with movement and changes in position of the hand. Thus, the wrist position cannot change. In practice, it is difficult to keep the pressure constant as a slight change in wrist pressure and sensor position affects readings to an appreciable extent. Furthermore, the calibration involves extrapolation and interpolation of readings. Therefore, user conditions must remain uniform, since one has to show a linear relationship which may not exist if user conditions are otherwise. In accordance with the described formula for calculating blood pressure, the pressure sensed by the piezoelectric film transducer is dependent on the area of contact, distance from the artery and source of the signal. These are factors which cannot practically be fixed with the described device.
To provide continuity in monitoring, the blood pressure must be measured on a beat-to-beat basis, as in intra-arterial monitoring.
The time-keeping function of a watch should be integrated with the blood pressure data, as this will provide a meaningful interpretation of the trend or pattern of blood pressure seen or recorded over a period of time. The downloading of data over time may become important in an unfortunate event of the death of a wearer.
Similarly, in the collection of data by the sensor, the position of the sensor and the fixation of the sensor must be considered. In order to accurately collect data from every beat of the heart, the sensor compartment must be able to receive reliable data with the wrist in different positions. In the prior art, the data can only be reliably collected when the hand is held fixed at a certain position, i.e. with restrictions. The prior art may try to overcome the movement of the strap by increasing the strap pressure. Usually, this is not only impractical, but undesirable as the compression of veins will cause significant congestion in the hand distal to it in just a few minutes. This can lead to numbness and further medical complications.
The Median Nerve at the Carpal Tunnel would be compressed causing numbness of the finger in a few minutes. As a result, the hand or fingers will swell, causing further congestion. This not only greatly affects the signal, but is harmful to the wearer. Therefore, the challenge is to be able to design the strap system that is comfortable to the wearer over a long period and holds the sensor in position well so as to allow for natural movement of the hand/wrist and collects the data accurately.
The donning and doffing of the wrist monitor and the whole calibration has to be simple and user-friendly for it to be of value for a person who is not medically trained.
However, there is overwhelming evidence in the past 3-4 years that demand us to take a new look at blood pressure monitoring. According to Professor Eoin O'Brien from the Beaumont Hospital, Dublin, Ireland, different individuals fall into distinctly different blood pressure patterns, which can only be identified by 24-hours tracings of the blood pressure (as opposed to single, momentary clinic/office reading). The 9 (not exhaustive) main blood pressure patterns identified are:                1. Normal Blood Pressure;        2. Borderline Hypertension;        3. Isolated Systolic Hypertension;        4. Isolated Diastolic Hypertension;        5. Systolic & Diastolic Hypertension with night time dip;        6. Systolic & Diastolic Hypertension without night time dip;        7. Nocturnal Hypertension;        8. White Coat Hypertension;        9. White Coat Normotension.        
Naturally, each individual pattern has its own risks and implications that require its unique management, which may or may not require pharmacological intervention. Without 24-hours blood pressure tracings, White Coat Hypertensive patients may unwittingly be put at increased risk due to unnecessary treatment. On the other hand, certain blood pressure patterns may predispose an individual to increased risk of a stroke or heart attack and early recognition of these patterns allow appropriate treatment to be given to arrest or slow the progression of the disease.
Against this medical background and clinical deficiency, the object of the present invention is to provide an improved device and method for continuous and non-invasive monitoring of arterial blood pressure.